Refusing to take Responsibilty for their MISTAKE

MTN is still failing to understand that i was on a TOP UP package at the time when they say i used data and made calls up to an amount of R2750.97, so i dont inactual fact owe them anything.They responded to my last post saying that i owe an amount of R2750.97. How can i owe them money when i was purchasing my own data and airtime. all of this mess up started during their strike. they need to write off that amount because i dont owe them anything at all and take full responsibility for their own mess up. I can't be held liable for their billing mistake. this is really starting to irritate me.and it seems i am not the only one with a billing complaint. Their service really sucks.